Secret Mark and the Long Ending of Mark

What do the rest of you think about Larry Hurtado?  Here's what I have to say - he certainly seems to develop the same argument for 'Secret Mark' being a modern forgery that he does later in the same book for the long ending of Mark being an ancient addition.  Can someone help me out what I'm not understanding about his point?  First Hurtado uses the pastiche argument to demonstrate why Morton Smith had to be the forger of the Letter to Theodore:

Secret Mark employs phrasing with uncanny resemblances to the canonical Gospels to narrate an incident that looks suspiciously like a novelistic expansion of the Markan narrative [Hurtado Lord Jesus Christ p. 436]

Then later on he uses another scholar's study of the long ending of Mark to demonstrate that these kinds of 'pastiche' compositions were en vogue.  Indeed it is how Hurtado says the ending of Mark was composed at the time of Irenaeus or earlier:

Most recently, James Kelhoffer's detailed analysis of the "long ending" of Mark shows that this block of material (16:9-20), which represents an attempt to fit Mark with a more "suitable" ending, used elements from the other three canonical Gospels, and these writings only. [p. 585]

So why does the pastiche argument (or cento gospel to use Grant's terminology) 'prove' that the story about the resurrected youth in to Theodore is a modern fake?  Please help me out ...